This invention relates to an apparatus for hardening a steel pipe, and more particularly, to an apparatus for quenching a hot steel pipe by providing longitudinally flowing coolant streams outside and inside the steel pipe.
As is well known in the art, an apparatus for hardening a steel pipe by quenching must meet the following requirements. From a functional aspect, (1) the cooling capacity must be sufficiently high or a sufficiently high rate of cooling must be ensured for thick-walled steel pipes and (2) the cooling rate must be constant over the entire length of steel pipes to prevent formation of soft spots. From an installation aspect, low initial investment and operating cost, easy maintenance, and easy adaptability to different diameters of steel pipes are necessary.
The prior art steel pipe hardening apparatus may be generally classified into two groups; one is the so-called ring-type hardening apparatus of the type wherein a plurality of high-pressure injection nozzles are circumferentially arranged about a steel pipe to inject a liquid coolant, for example, cooling water under pressure toward the outer surface of the steel pipe, and the other is the so-called immersion hardening apparatus of the type wherein a steel pipe is introduced and immersed in a liquid coolant, for example, cooling water in a cooling tank. The ring-type hardening apparatus have disadvantages that the cooling capacity is lower as compared with the immersion hardening apparatus and the inner surface of a thick-walled steel pipe experiences a reduced rate of cooling because in general, only the outer surface is cooled with water. In order to achieve an extra cooling at the inner surface of a steel pipe in addition to outside cooling in the ring-type hardening apparatus, it has been practised to insert a header having an injection nozzle into the steel pipe. However, this method is difficult to apply to steel pipes having a relatively small inner diameter. Since insertion and removal of the header into and out of a steel pipe must be repeated for each steel pipe to be hardened, the time required for insertion and removal of the header becomes a limiting factor in increasing the total throughput speed during the successive hardening of a number of steel pipes, resulting in a limited throughput capacity.
On the other hand, the immersion hardening apparatus generally includes means for forcedly agitating a liquid coolant such as cooling water to produce a forced water flow in the cooling tank because spontaneous convection of water only results in a reduced cooling capacity. When a hot steel pipe is introduced into the cooling tank and immersed in cooling water, the transfer of heat energy from the steel pipe surface to the adjoining layer of cooling water causes the cooling water to boil to cover the steel pipe surface with a film of steam. Inconveniently, the formation of a steam film results in a considerable reduction in rate of heat transfer between the steel pipe and the cooling water or rate of cooling. As the steam film disperses away from the steel pipe surface, direct heat transfer is established again between the steel pipe and cooling water and convection cooling starts. If the dispersion of a steam film from the steel pipe surface is delayed, the rate of cooling of the steel pipe is reduced below the critical cooling rate for martensite transformation required in normal hardening, failing to achieve effective hardening. It it thus critical for the immersion hardening apparatus that a steam film formed at the steel pipe surface be removed as rapidly as possible to start cooling by ordinary heat transfer and convection. To this end, it is necessary to expose the steel pipe surface to a cooling water stream having a relatively high flow velocity. Such a high-velocity cooling water stream may be produced in the conventional immersion hardening apparatus by means of an arrangement shown in FIG. 20. Referring to FIG. 20, a steel pipe 2 is placed in a cooling tank 1. A plurality of spaced-apart injection nozzles 3 are circumferentially arranged about the steel pipe 2 such that they inject cooling water under high pressure tangentially of the steel pipe 2 to produce an agitating stream 4 circumferentially flowing along the outer surface of the steel pipe 2. In combination of the forced stream flowing outside the steel pipe 2, a longitudinally flowing water stream is produced inside the steel pipe 2 by means of an axial injection nozzle (not shown) at one end of the steel pipe. However, such advanced immersion hardening apparatus still have many problems, particularly associated with the means for producing a forcedly agitating stream.
In producing an agitating stream having a sufficient flow velocity to attain effective quenching, the kinetic energy of a jet stream injected through an injection nozzle is transmitted to static water in the cooling tank to cause the static water to move. Because of low energy efficiency, the injection pressure and flow rate must be undesirably increased. Since a number of injection nozzles must be arranged at small intervals in the longitudinal direction of a steel pipe in order to quench the steel pipe uniformly over its entire length, the apparatus becomes more complicated and expensive. Furthermore, injection nozzles arranged about a steel pipe tend to be blocked with scales such as chips of an oxide coating peeling from the steel pipe surface as well as deposits from water, and as a result, the cooling capacity is locally reduced to form soft spots. In order to effectively cool a steel pipe from its outside by producing a circumferentially flowing water stream along the outer surface of the steel pipe, the width of a support for supporting the steel pipe in the cooling tank, more specifically, the width of a support in the longitudinal direction of the steel pipe should be small enough to reduce the resistance to the circumferentially flowing stream by the support. With a reduced width of the support, the steel pipe will experience an increased impact stress when it is thrown into the cooling tank and falls to the support. The steel pipe is often impaired at the surface by such collision. Another problem is to discharge heated water. In the above-described prior art immersion hardening apparatus, the cooling water which has completed quenching of the steel pipe is discharged by allowing it to pass an overflow weir of the cooling tank. However, the prior art immersion hardening apparatus of the above-described construction is difficult to selectively discharge only the heated cooling water, resulting in a reduced rate of cooling.
An apparatus for hardening a long steel pipe is disclosed in Franceschina et al. U.S. Pat. No. 3,877,685 (issued Apr. 15, 1975). The steel hardening apparatus of this U.S. Patent comprises a container dimensioned to receive a hot steel pipe to be hardened, means for supporting the hot pipe in a predetermined position in the container, a nozzle for introducing cooling water into the pipe, means for moving the nozzle between a retracted position in which the tip thereof is spaced from one end of the pipe and an expanded position in which the tip lies within the one end of the pipe, inlet means for introducing cooling water into the container so as to pass into and around the pipe, and isolator means movable in relation to the nozzle. The tip of the nozzle is inserted into the end of the pipe received in the container before cooling water is supplied into the container through the inlet means so as to pass into and around the pipe. The isolator means may be moved to regulate the flow rate of cooling water flowing outside the pipe.
Although the above-mentioned apparatus allows cooling water to pass into and around a steel pipe to be hardened, the steel pipe is simply located and supported in the container. Since no flow path is defined outside the steel pipe for the passage of cooling water it cannot be expected that cooling water supplied around the pipe will flow parallel to the central axis of the pipe to the back end of the pipe. Rather, a turbulent flow is often induced and particularly, the flow velocity varies in the circumferential dirction because the outside flow path is open or it forms an open channel. The turbulent flow and varying flow velocity will cause serious problems. The steel pipe would be locally covered with a film of steam resulting from evaporation of cooling water, and/or heated cooling water which has taken up heat from the steel pipe would stagnate on some part of the steel pipe. As a result, the steel pipe is not uniformly quenched over its entire length, resulting in formation of soft spots and deformation, particularly extreme bending of the pipe. The isolator means is moved in relation to the nozzle to regulate the flow rate of cooling water flowing outside the steel pipe. In addition, the above-mentioned apparatus is complicated and expensive as a whole.
The present invention is based on the recognition that the method for hardening a hot steel pipe by supplying cooling water so as to pass into and around the steel pipe is advantageous over the prior art methods. The inventors have completed the present invention through further researches to develop an apparatus for carrying out this method under optimum conditions.
It is, therefore, a primary object of the present invention to provide an apparatus for hardening a steel pipe by injecting a liquid coolant so as to pass into and around the steel pipe in the longitudinal direction thereof whereby thick-walled steel pipes can be uniformly hardened without forming soft spots and cracks.
It is another object of the present invention to provide a steel pipe hardening apparatus in which a steel pipe to be hardened is received in a cylindrical assembly which can be opened or closed for insertion and removal of the steel pipe, and cooling water flows through flow paths defined outside and inside the steel pipe in the cylindrical assembly.
A further object of the present invention is to provide a steel pipe hardening apparatus of the above-mentioned type in which the cylindrical assembly is opened to prevent damage to the cylindrical assembly and/or the steel pipe when the steel pipe extremely bends within the cylindrical assembly.
A still further object of the present invention is to provide a steel pipe hardening apparatus in which the inner diameter of the cylindrical assembly can be changed so as to match with the outer diameter of a steel pipe to be hardened.
An additional object of the present invention is to provide a steel pipe hardening apparatus capable of continuously hardening steel pipes.